1. Field of the Invention
The present invention relates to a silver halide photographic emulsion and, more particularly, to a silver halide photographic emulsion containing an organic thioether compound.
2. Description of the Prior Art
In the production of silver halide photographic emulsions, it has long been known to use organic thioether compounds as solvents for silver halide or as chemically sensitizing agents.
For example, U.S. Pat. Nos. 3,271,157, 3,531,289, 3,574,628, 4,057,429, etc., describe the techniques of producing a so-called monodisperse emulsion containing silver halide grains of a uniform size, by allowing an organic thioether compound to coexist in the silver halide (AgX) formation step or upon physical ripening for producing silver halide photographic emulsions (hereinafter merely referred to as emulsion).
Also, U.S. Pat. Nos. 2,521,926, 3,021,215, 3,038,805, 3,506,443, 3,057,724, 3,062,646, 3,574,709, 3,622,329, 3,625,697, etc., describe the techniques for raising photographic sensitivity of the emulsion by allowing an organic thioether compound to exist upon chemical ripening for producing the emulsion or immediately before coating.
However, existence of an organic thioether compound upon preparation of the emulsion tends to increase fog.
Although it is known that addition of conventionally known thioether compounds serves to enhance photographic sensitivity, the degree of increase in photographic sensitivity in the case of exposing with a comparatively high illuminance for a short time (for example, 1/100 to 1/1,000 second) is about the same as that in the case of exposing with a low illuminance for a long time (for example, 5 to 10 seconds) and, as opposed to the present invention, the reciprocity law failure in low illuminance region has never been improved. (The term "reciprocity law failure" as used in the photographic science refers to the fact that photographic sensitivity changes with exposure time, and improvement in reciprocity law failure indicates that the change in photographic sensitivity with exposure time is reduced.)
In order to prevent the fog caused by the organic thioether compounds, it has been known, for example, to conduct Agx formation at a pH of not more than 4, or to super-purify the organic thioether compounds. However, these techniques have such defects that sufficient effects cannot be attained and they cannot be practically conducted on an industrial scale.